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In population genetics, 'genetic load' or 'genetic burden' is a measure of the cost of lost alleles due to selection ('selectional load') or mutation ('mutational load'). It is a value in the range , where 0 represents no load.
The concept was first formulated in 1937 by JBS Haldane, independently formulated, named and applied to humans in 1950 by H. J. Muller^[1], and elaborated further by Haldane in 1957.^[2]

Contents

Definition

Mathematics

Causes of genetic load

Mutational load

Selectional load

Segregational load

Creationist criticism

References

External links

Definition

Genetic load is the reduction in selective value for a population compared to what the population would have if all individuals had the most favored genotype.^[3] It is normally stated in terms of fitness as the reduction in the mean fitness for a population compared to the maximum fitness.

Mathematics

Consider a single gene locus with the alleles $mathbf\{A\}\; \_1\; dots\; mathbf\{A\}\; \_n$, which have the fitnesses $w\_1\; dots\; w\_n$ and the allele frequencies $p\_1\; dots\; p\_n$ respectively. Ignoring frequency-dependent selection, then genetic load ($L$) may be calculated as:
:
where $w\_max$ is the maximum value of the fitnesses $w\_1\; dots\; w\_n$ and is mean fitness which is calculated as the mean of all the fitnesses weighted by their corresponding allele frequency:
:
where the $i^mathrm\{th\}$ allele is $mathbf\{A\}\_i$ and has the fitness and frequency $w\_i$ and $p\_i$ respectively.
When the $w\_max\; =\; 1$, then (1) simplifies to
:

Causes of genetic load

Load may be caused by selection and mutation.

Mutational load

Load caused by mutations is known as mutational load.

Selectional load

Selection occurs when the fitnesses of particular alleles are inequal, hence selection always exerts a load.
With directional selection, the allele frequencies will tend towards an equilibrium position with the fittest allele reaching a frequency in mutation-selection balance. As mutations are rare, this is effectively fixation. Consider two alleles $mathbf\{A\}\_1$ and $mathbf\{A\}\_2$. If $w\_1\; >\; w\_2$, then at equilibrium, and , hence , and .
If the mean fitness is 0, the load is equal to 1, but the population goes extinct.

Segregational load

In contrast to directional selection, heterozygote advantage always exerts a load at equilibrium.

Creationist criticism

Some creationists (such as Henry M. Morris) have suggested that mutational load would increase over time and thus make populations inviable. However, they ignore the effect of selectional load acting to weed out (decrease frequency of) deleterious mutations.

References

1.
2. The cost of natural selection, JBS Haldane, , , Journal of Genetics, 1957
3. Some possibilities for measuring selection intensities in man, JF Crow, , , Hum. Biol, 1958

External links

★ ''The Cost of Natural Selection Revisited'', Leonard Nunney, Ann. Zool. Fennici 40:185-194 (pdf file)

★ Genetic load, from ''Evolution A-Z'' by Mark Ridley

★ "Understanding Genetic Load" - a paper by Dr. L. Monroe

★ Creationist Claim CB120: Genetic load from ''An Index to Creationist Claims'' by Mark Isaak